Investigation on the synthesis and characterization of amorphous steels
- Autores: Fuqiang Zhai
- Directores de la Tesis: Daniel Crespo Artiaga (dir. tes.), María Jazmin Duarte Correa (codir. tes.)
- Lectura: En la Universitat Politècnica de Catalunya (UPC) ( España ) en 2015
- Idioma: español
- Tribunal Calificador de la Tesis: Amadeu Concustell i Fargas (presid.), Pere Bruna Escuer (secret.), Frank U. Renner (voc.)
- Materias:
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- Resumen
Fe-based amorphous steels are promising materials for engineering applications due to their high strength, high hardness, and good soft-magnetic and corrosion properties. However, the low glass forming ability (GFA) has impeded their practical applications. Through great number of previous research works, it was proved that minor addition is an effective and widely used method to improve the GFA and enhance the properties of the developed metallic glasses. Thus, in this thesis we used the minor addition method to develop new Fe-based amorphous steel with relatively higher GFA, and the mechanism of composition selection and optimization were investigated in order to improve the glass-forming ability and to produce larger sized Fe-based amorphous steel. Moreover, the corrosion and micro-mechanical properties of the developed Fe-based amorphous steel were studied.
Firstly, a new Fe-based bulk metallic glass with superior GFA, Fe46Cr15Mo14C15B6Nb4, was developed based on the Fe-Cr-Mo-C-B alloy system by minor addition of Nb. It was found that the optimum addition content of Nb was of 4 at.% . A fully amorphous rod sample with 3 mm in diameter was produced. This alloy shows an ultimate compressive strength of 1920 MPa and Vicker¿s hardness 1360 HV. The crystallization kinetics studies found that the activation energies for glass transition, onset of crystallization and crystallization peak were higher than those of other reported Fe-based bulk metallic glasses. The value of the fragility parameter m for the Fe46Cr15Mo14C15B6Nb4 alloy was calculated to be 34, indicating that the Fe-Cr-Mo-C-B-Nb alloy system is a strong glass former. It is inferred that the more sequential change in the atomic size, the generation of new atomic pairs with large negative heats of mixing and the amount of oxygen in the molten liquid neutralized into Nb oxides provide a synergetic effect for the remarkably improved GFA and thermal stability.
Secondly, the corrosion resistance properties of the Fe50Cr15Mo14C15B6 alloy with different contents of Nb were investigated. It was found that the corrosion resistance of the Fe-Cr-Mo-C-B alloys in 0.1M H2SO4 solution could be improved by adding appropriate content of Nb. The measured open circuit potential OCP and calculated corrosion potential Ecorr moved to more anodic potentials when adding 4 at.% Nb, reaching values of 0.408 and 0.391 V, respectively. The corrosion current density icorr and passive current density ipass gradually decrease and attain a minimum of 4.71×10-6 and 1.25 ×10-5 A/cm2 respectively when adding 4 at.% Nb. The produced alloy samples were spontaneously passivated with significantly low passive current density in the range of 10-20 µA/cm2 with a relatively wide passivation region. The chemically and structurally homogeneous atomic configuration of metallic glasses, and the rapid formation of uniform, chromium-enriched passive films, contribute to the excellent corrosion resistance of Fe-Cr-Mo-C-B-Nb amorphous alloys.
Finally, the micro-mechanical properties of the produced amorphous steel were investigated by nanoindentation. It was found that the hardness and Young modulus of the produced amorphous steel eventually tended to a constant value with increasing the depth of indentation. The hardness and Young modulus of the produced amorphous alloys gradually enhance with increasing the content of Nb. By adding 6 at.% Nb maximums of hardness and Young modulus of 16.1±1.0 and 243±22 GPa, respectively, were obtained. Through adding Nb to the Fe50Cr15Mo14C15B6 base alloy. The observed shear bands after nanoindentation were larger in Nb-containing samples. This results from the relative higher Young modulus of the Fe-Cr-Mo-C-B-Nb amorphous alloys, probably due to a close atomic packing and more homogeneous atomic structure.
